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Abstract:

Provided is a Schottky emitter having the conical end with a radius of
curvature of 2.0 μm on the emission side of an electron beam. Since a
radius of curvature is 1 μm or more, a focal length of an electron gun
can be longer than in a conventional practice wherein a radius of
curvature is in the range of from 0.5 μm to not more than 0.6 μm.
The focal length was found to be roughly proportional to the radius of
the curvature. Since the angular current intensity (the beam current per
unit solid angle) is proportional to square of the electron gun focal
length, the former can be improved by an order of magnitude within a
practicable increase in the emitter radius. A higher angular current
intensity means a larger beam current available from the electron gun and
the invention enables the Schottky emitters to be used in applications
which require relatively high beam current of microampere regime such as
microfocus X-ray tube, electron probe micro-analyzer, and electron beam
lithography system.

Claims:

1. An electron beam control method comprising an electron beam generating
step of emitting electrons from the conical end of an emitter sharpened
in the shape of a cone on the emission side of an electron beam by using
a Schottky effect under an electric field applied to the conical end to
thereby generate an electron beam, the method further comprising:a
curvature radius adjusting step of adjusting a radius of curvature of the
conical end;a focal length control step of controlling a focal length of
an electron beam by a radius of curvature adjusted in the curvature
radius adjusting step;a angular current density control step of
controlling a angular current density of an electron beam with a focal
length controlled by the focal length control step, whereinthe electron
beam generating step is conducted each time emission of an electron beam
in a state where a angular current density is controlled after the
angular current density control step.

2. The electron beam control method according to claim 1, whereinthe
radius of curvature is selected in the range of 1 μm or more in the
curvature radius adjusting step.

3. The electron beam control method according to claim 1, further
comprising:a protrusion length adjusting step of adjusting a protrusion
length that is a length of the conical end from a suppressor electrode
which is on the side opposite the emission side among the two electrodes
establishing an electric field and to which a negative voltage is applied
when the conical end is protruded on the emission side outward from the
suppressor electrode; anda combination range setting step of setting a
combination range of the protrusion length and the radius of curvature
based on the value of the electric field, whereinin the protrusion length
adjusting step, a protrusion length is selected in the combination range
at a radius of curvature adjusted in the curvature radius adjusting step
based on the combination range.

4. The electron beam control method according to claim 3, whereinthe
radius of curvature is selected in the range of 1 μm or more and 4
μm or less in the curvature radius adjusting step, anda protrusion
length is selected in the range of 200 μm or more and 1500 μm or
less from the combination range at the radius of curvature adjusted in
the curvature radius adjusting step based on the combination range and
the radius of curvature.

5. The electron beam control method according to claim 1, further
comprising:an emitter forming step of adjusting the protrusion length of
the conical end and, also, applying the emitter with forming not
revealing a (100) crystal plane in the side surface portion of the
emitter on the emission side outward from the suppressor electrode that
is applied with a negative voltage and located on the side opposite the
emission side among the two electrodes applying the electric field.

6. An electron beam generating apparatus comprising: an emitter having the
conical end sharpened in the shape of a cone on the emission side of an
electron beam; and two electrodes applying an electric filed to the
conical end of the emitter, wherein the electric field is applied to the
conical end to thereby emit electrons using a Schottky effect, so that an
electron beam is generated,the electron beam generating apparatus having
an improvement that a radius of curvature of the conical end is 1 μm
or more.

7. The electron beam generating apparatus according to claim 6, whereina
suppressor electrode and the emitter are disposed so that a protrusion
length, that is a length to the topmost point of the conical end from the
suppressor electrode, is in the range of 200 μm or more and 1500 μm
or less when the conical end is protruded on the emission side outward
from the suppressor electrode, wherein the suppressor electrode is on the
side opposite the emission side among the two electrodes establishing an
electric field and carry a negative voltage, anda radius of curvature of
the conical end is in the range of 1 μm or more and 4 μm or less.

8. The electron beam generating apparatus according to claim 6, whereinthe
emitter has a form not to reveal a (100) crystal plane in the emitter
side surface portion on the emission side outward from a suppressor
electrode, located on the side opposite the emission side of among the
two electrodes establishing an electric field, and carry a negative
voltage.

9. An apparatus using electron beam generating means including: an emitter
having the conical end sharpened in the shape of a cone on the emission
side of an electron beam; and two electrodes applying an electric filed
to the conical end of the emitter, wherein the electric field is applied
to the conical end to thereby emit electrons using a Schottky effect, so
that an electron beam is generated, the electron beam generating means
having an improvement thata radius of curvature of the conical end is 1
μm or more,the apparatus further comprising:processing means
conducting a predetermined processing based on an electron beam generated
by the electron beam generating means.

10. The apparatus using electron beam generating means according to claim
9, whereina suppressor electrode and the emitter are disposed so that a
protrusion length, that is a length to the topmost point of the conical
end from the suppressor electrode, is in the range of 200 μm or more
and 1500 μm or less when the conical end is protruded on the emission
side outward from the suppressor electrode, wherein the suppressor
electrode is on the side opposite the emission side among the two
electrodes establishing an electric field and carry a negative voltage,
anda radius of curvature of the conical end is in the range of 1 μm or
more and 4 μm or less.

11. The apparatus using electron beam generating means according to claim
9, whereinthe emitter has a form not to reveal a (100) crystal plane in
the emitter side surface portion on the emission side outward from the
suppressor electrode, located on the side opposite the emission side
among the two electrodes establishing an electric field, and carry a
negative voltage.

12. The apparatus using electron beam generating means according to claim
9, whereinthe apparatus is an electron probe microanalyzer conducting an
analysis or observation of a specimen, andthe processing means conducts
an analysis or observation of a specimen by irradiating the specimen with
an electron beam to obtain an X ray image based on X rays generated from
the specimen, or by irradiating a specimen with an electron beam to
obtain an electron beam image based on secondary electrons or reflected
electrons generated from the specimen.

13. The apparatus using electron beam generating means according to claim
9, whereinthe apparatus is an X ray tube, andthe processing means is a
target generating X rays by collision with an electron beam.

15. An emitter generating an electron beam in which the conical end of an
emitter on the emission side of an electron beam is sharpened in the
shape of a cone and is applied with an electric field to thereby emit
electrons using a Schottky effect, the emitter having an improvement
thata radius of curvature of the conical end is 1 μm or more.

16. The emitter according to claim 15, whereina radius of curvature of the
conical end is 1 μm or more and 4 μm or less.

17. The emitter according to claim 15, which has a form not to reveal a
(100) crystal plane in the emitter side surface portion.

Description:

BACKGROUND OF THE INVENTION

[0001](1) Field of the Invention

[0002]The invention relates to an electron beam control method, an
electron beam generating apparatus, a device using the same, and an
emitter.

[0003](2) Description of the Related Art

[0004]Electron guns in electron beam based instruments use two types of
cathodes (emitters); a thermionic emitter and a field emitter. A
thermionic emitter uses a tungsten filament, a pointed emitter of a
single crystal or a sintered compound of lanthanium hexaboride
(LaB6) or cerium hexaboride (CeB6). The emitter is heated and
caused to emit thermal excited electrons to thereby generate an electron
beam. A field emitter uses a sharpened conical end of an electrode on the
emission side of an electron beam and emits electrons by using a
tunneling effect or a Schottky effect caused by a strong electric field
applied to the conical end to thereby generate an electron beam.

[0005]Note that in a case where an analysis or observation is carried out
in a small region, an electron beam with a high brightness is required in
order to reduce a diameter thereof (here, the "brightness" is defined as
the current density per unit solid angle of the electron beam).
Therefore, in recent years, a field emitter has been adopted, instead of
a thermionic emitter that has been conventionally employed, in a scanning
electron microscope (hereinafter also referred to as "SEM" for short) and
an electron probe microanalyzer (hereinafter also referred to as "EPMA"
for short) as well as other electron beam based instruments; transmission
microscopy, electron beam lithography, inspection tools, etc. in analysis
or observation in a small region to thereby improve a spatial resolution.

[0006]There are two types of field emitters, a cold field emitter and a
thermal field emitter. In the case of a cold field emitter, the conical
end of an emitter is normally made from a single crystal fine tungsten
wire and is subjected to a strong electric field at room temperature
whereby electrons, in the single crystal, are emitted using a tunneling
effect, so that an electron beam is generated. In the case of a thermal
field emitter, the conical end of an emitter made from a single crystal
fine tungsten wire coated with zirconium oxide (ZrO) is heated while
being subjected to a strong electric field which causes electrons to be
emitted using a Schottky effect, so that an electron beam is generated.
Since a thermionic emitter uses a Schottky effect as described above, it
is also called a Schottky emitter.

[0007]In a Schottky emitter, a zirconium oxide layer coating the conical
end of the emitter has an effect of reducing a work function of a crystal
face, formed in the conical end, and which is a (100) crystal plane.
Therefore, a uniform, strong electron beam is emitted and extracted from
the conical end. Note that a Schottky emitter technology is disclosed in
U.S. Pat. Nos. 145,042 and 145,043.

[0008]In the case of a field emitter, as described above, the current
density is, however, higher than that of a thermionic emitter. In the
case of a field emitter, the electron source diameter, where an electron
beam is emitted from in an electron gun configuration, is very small, as
shown in FIG. 9B, in comparison with a thermionic emitter of FIG. 9A
(FIG. 9B shows a Schottky emitter). An electron source diameter is
several tens of μm in a case of a thermionic emitter, while in a field
emitter represented by a Schottky emitter, an electron source diameter is
several tens of nm. If an electron source area of a thermionic emitter is
indicated by dSTE, and an electron source area of a field emitter is
indicated by dSFE, the areas are different from each other by up to
six orders of magnitude.

[0009]On the other hand, if a solid angle of an electron beam is indicated
by dΩ and a beam current value (current value is indicated by
Ib), a solid angle dΩ of the electron beam varies according to
a beam current value Ib to be required. If an axial brightness of
the electron beam is indicated by B, a beam current value Ib is
given by the following equation (1) with an electron source area dS and a
solid angle dΩ.

Ib=B×(dS×dΩ) (1)

[0010]In a case where a larger beam current is necessary, it is understood
from equation (1) that an effective solid angle dΩ increases for
fixed brightness and source area.

[0011]A Schottky emitter is much higher in brightness than a thermionic
emitter (by about three orders of magnitude). However, since an electron
source area dSFE is smaller than dSTE by up to six orders of
magnitude, a solid angle dΩ of an electron beam in a case where the
same beam current is secured is larger in a Schottky emitter than that in
a thermionic emitter. That is, if a solid angle of an electron beam in a
thermionic emitter is indicated by dΩTE and a solid angle of
an electron beam in a field emitter represented by a Schottky emitter is
indicated by dΩFE, a relation expressed by the following
equation (2) is established.

dΩFE>dΩTE (2)

[0012]That is, an angular current density which is the current per unit
solid angle for a Schottky emitter is smaller than that of a thermionic
emitter although the Schottky emitter has a higher axial brightness than
the thermionic emitter.

[0013]Since, with a larger solid angle, an electron beam is diverged,
collimation is required. As a result, in a field emitter, an aberration
in an accelerating and condenser lens section downstream from the
emission side exerts a large influence, so that a characteristic of the
emitter, which would be by nature high in brightness, is degraded by an
influence of the aberration, and the "apparent brightness" decreases as a
beam current increases. FIG. 10 is a graph showing relationships between
a beam current value and brightness in the case where a Schottky emitter
is employed as an example of a field emitter and the case where a
tungsten filament emitter is employed as an example of a thermionic
emitter. The abscissa is assigned to a beam current and the ordinate is
used for plotting brightness. A dotted line is a curve concerning a
tungsten filament emitter and a solid line shows a curve concerning a
Schottky emitter. Note that in the Schottky emitter, the curve was
obtained under the conditions where the emission current density js
is 1.0×104 A/cm2, an emitter temperature T is 1800 K and
an angular current density J.sub.ΩSE is 0.429 mA/str, while for the
thermionic emitter, the curve was obtained under the conditions where the
emission current density js is 3 A/cm2, an emitter temperature
T is 2800 K and an angular current density J.sub.ΩW=140 mA/str. The
term "W filament" indicates the tungsten filament operated in the
thermionic mode and the term "SE" indicates the Schottky emitter.

[0014]In a case of a thermionic emitter represented by a tungsten
filament, the angular current density is high; therefore a decrease in
brightness is not problematical in a practical aspect giving a reduced
brightness when the current is in the neighborhood of a value in the
range of 10 μA to 20 μA. On the other hand, in a case of a field
emitter represented by a Schottky emitter, an angular current density is
lower and an electron source diameter is smaller; therefore, the
brightness begins to decrease when the beam current is in the
neighborhood of 1 nA and the brightness decreases by 6 orders of
magnitude at a beam current 1 μA.

[0015]Since a beam current employed in a case of a scanning electron
microscope (SEM) is at a level of nA or less, no reduction in brightness
is observed with a Schottky emitter in a case where the emitter is used
in SEM. Thus, a Schottky emitter can be used in SEM. However, in a case
of a device where a beam current at a level of sub μA or μA is
required as in a electron probe micro analyzer (EPMA), reduction in the
brightness is observed at a level of sub μA or μA for a Schottky
emitter; therefore, even if a Schottky emitter is employed in instruments
such as EPMA, only an electron beam with a low brightness can be used.
Hence, it is impossible to employ a field emitter in instruments such as
EPMA in a practical sense.

SUMMARY OF THE INVENTION

[0016]The invention has been made in light of such circumstances and it is
an object of the invention to provide an electron beam control method, an
electron beam generating apparatus, capable of freely setting the angular
current density in a Schottky emitter.

[0017]The following findings and knowledge have been obtained in order to
achieve such an objective.

[0018]As shown in FIG. 11A, a Schottky emitter 201 has a construction in
which a conical end 201a of an emitter 201 on an emission side B of an
electron beam is, as described above, sharpened in the shape of a cone.
Note that FIG. 11B is a schematic diagram in which the conical end 201a
thereof is enlarged and if a radius of curvature of the conical end 201a
is indicated by R, R is in the range of R<0.5 μm to 0.6 μm.

[0019]Note that if trajectories of electron beams emitted from an electron
gun (emitter) are called as "cathode trajectories", a primary
characteristic of the cathode trajectories is characterized by electron
gun focal length. (S. Fujita and H. Shimoyama, J. Electron Microscopy,
54(4), 331-343 (2005)) FIG. 12 is a diagram schematically showing the
emitter (cathode) of an electron gun. If an angle is formed between an
electron trajectory emitted normal to (at a given angle relative to) the
cathode surface at position ξ and an optical axis on a reference place
(a drift region) is indicated by β as shown in FIG. 12, a focal
length f is defined by the following differential equation (3).

1/f=-(∂ sin β/δξ)|u=0ξ→0 (3)

[0020]It is seen in the equation (3) that the reciprocal of the electron
gun focal length is the limiting ratio of the sine of the emerging ray
angle to the off-axis distance of the starting position for the electrons
emitted perpendicularly to the cathode surface. A crossover diameter, the
minimum beam diameter of an electron beam formed along the optical axis
(an electron source diameter) and an angular current density are obtained
from the focal length f defined by the equation (3).

[0021]If an electron source diameter is indicated by dco, a
Boltzmann's constant k, an absolute temperature T, the electronic charge
e, a potential (an extraction potential) at an extractor electrode
Vext and a current density at a cathode (a cathode current density)
js, then an electron source diameter dco and an angular current
density J.sub.Ω are given by the following equations (4) and (5),
respectively.

dco=2×f×{(k×T)/(e×Vext)}1/2 (4)

J.sub.Ω=f2×js (5)

[0022]If a focal length f is longer, an electron source diameter dco
is larger as is understood from the equation (4) and an angular current
density J.sub.Ω is also raised as is understood from the equation
(5). Hence, in order to set an angular current density J.sub.Ω in a
Schottky emitter with reasonable freedom, it is only required to adjust a
focal length f.

[0023]Note that the increase (or decrease) of the angular current density
J.sub.Ω by the change in the electron gun focal length necessarily
accompanies the increase (or decrease) of the electron source diameter
dco. Consequently the brightness B itself is independent of the
focal length as is shown below,

B=J.sub.Ω/π(dco/2)2=(1/π)(e js/kT)Vext,
(6)

Hence, in order to control both of the electron optically important
parameters, i.e. the brightness B and the angular current density
J.sub.Ω, it is necessary to have simultaneously under control the
electron gun focal length f and the cathode current density js.

[0024]Let's start with the electron gun focal length. Then, attention will
be paid to the equation (3), which defines a focal length f. It was found
in this invention that the electron gun focal length can be adjusted by
altering the shape of the Schottky emitter. By scaling up or down the
emitter tip radius R it is possible to increase or decrease the off-axis
distance ξ which corresponds to a fixed emerging ray angle β. A
focal length f defined by the equation (3) is obtained by fitting v (=sin
β) in FIG. 12 using the following equation (6).

v=(-1/f)×ξ+ε×ξ3 (7)

[0025]FIG. 13 is a graph showing relationships between ξ and v in cases
where two Schottky emitters with different radii of curvature R were
employed. The abscissa is assigned to ξ and, also, the ordinate is
used for plotting v. To be concrete, employed here is a Schottky emitter
formed with a radius of curvature R of a conventional dimension of 0.6
μm and a Schottky emitter formed with a radius of curvature of 2.0
μm, which is larger than conventional size cathodes. As shown in FIG.
13, a curve of a conventionally standard Schottky emitter (R=0.6 μm)
is marked with the term "standard SE", while a curve of a Schottky
emitter (R=2.0 μm) having a radius of curvature R larger than the
conventional size is marked with the term "Giant SE".

[0026]In FIG. 13, inclinations of both curves in the vicinity of ξ=0
take a value (-1/f). By comparison in inclination, the Giant SE with a
smaller inclination has a focal length f longer than the standard SE with
a larger inclination.

[0027]As described above, findings and knowledge have been obtained that
by adjusting the radius of curvature of the conical end of an emitter one
can control a focal length and consequently a angular current density can
be freely set. In particular, findings and knowledge have been obtained
that if a radius of curvature of the conical end of an emitter is
adjusted to be larger than those employed for conventional Schottky
emitters, the focal length becomes longer and consequently the angular
current density can be increased.

[0028]Next, we shall investigate the cathode current density. With
Schottky emitters and cold field emitters the current density js is
a function of the electric field strength at the cathode. Since in the
case of the point cathode tip the electric field is enhanced by its small
curvature radius, the change in the radius usually influences the field
strength. Larger emitters would have a lower field if the other electrode
configurations and the applied voltage were unchanged. Because the
applied voltages cannot be augmented indefinitely without risk of the
discharge, some compensation in the electrodes configuration is necessary
in order to recover a high enough electric field with larger radius
emitters. One effective way of its realization is to make the protrusion
length of the emitter from the suppressor longer in the Schottky emitter
module configuration which consists of an emitter, a suppressor and an
extractor. By setting an appropriate protrusion length in accordance with
the emitter tip radius it is possible to secure high enough tip field
under reasonable applied voltage condition.

[0029]Therefore, the invention reported here based on the findings and
knowledge obtained by the inventors has the following configuration.

[0030]That is, an electron beam control method related to the invention is
an electron beam control method of generating an electron beam from an
electron emitter by Schottky effect by applying an electric field to said
emitter, wherein said electron emitter comprises a sharp tip of conical
shape and the method comprises the step of adjusting a radius of
curvature of said tip, thereby to control the focal length of an electron
beam emitted from said tip and thereby control the angular current
density of said beam.

[0031]A preferable example of the electron beam control method related to
the invention is to select a radius of curvature in the range of 1 μm
or more. By selecting a radius of curvature in the range of 1 μm or
more, a focal length is controlled so as to be longer than in a
conventional case where a radius of curvature is in the range from 0.5
μm to no more than 0.6 μm and, besides, a angular current density
can be controlled to a higher value than in the conventional case.

[0032]Preferably, the electron beam control method related to the
invention comprises:

[0033]a protrusion length adjusting step of adjusting a protrusion length
that is a length of the conical end from a suppressor electrode which is
located on the side opposite to the emission direction among the two
electrodes applying an electric field and to which a negative voltage is
applied; and

[0034]a combined range setting step of setting a combined range of the
protrusion length and the radius of curvature based on the value of the
electric field, wherein

[0035]in the protrusion length adjusting step, a protrusion length is
selected in the combined range at a radius of curvature adjusted in the
curvature radius adjusting step.

[0036]According to the electron beam control method of selecting a
protrusion length, a protrusion length is adjusted to thereby enable an
electric field strength at the conical end to be controlled. Hence, in a
Schottky emitter, a protrusion length is adjusted so as to enable a
necessary electric field to be secured. A characteristic of an electric
field with the protrusion length also changes according to an emitter's
radius of curvature. Therefore, a combined range of a protrusion length
and a radius of curvature are set in the combined range setting step
based on a value of an electric field. A protrusion length is selected at
a radius of curvature adjusted in the curvature radius step within the
combined range. The selection enables a protrusion length to be adapted
for a radius of curvature.

[0037]A preferable example of the electron beam control method selecting a
protrusion length is that the radius of curvature is selected in the
range of 1 μm or more and 4 μm or less in the curvature radius
adjusting step, and a protrusion length is selected in the range of 200
μm or more and 1500 μm or less from the combined range at the
radius of curvature adjusted in the curvature radius adjusting step (see
FIG. 4), thereby enabling a controlled increase of the angular current
density to higher than conventional values to be realized while
maintaining the high beam brightness of the Schottky emitter by ensuring
the high cathode current density js with the appropriate electric
field at the tip.

[0038]Preferably, the electron beam control method related to the
invention comprises: an emitter forming step of adjusting the protrusion
length of the conical end and, also, forming the emitter to avoid
revealing a (100) crystal plane on the lateral surface of the emitter on
the emission side outward from the suppressor electrode that is applied
with a negative voltage and located on the side opposite the emission
side among the two electrodes applying the electric field.

[0039]According to the electron beam control method applying forming not
to reveal a (100) crystal plane in the side surface portion of an emitter
on the emission side outward from the suppressor electrode, an
unnecessary (100) plane is hidden in the rear part on the side opposite
the emission side of the suppressor electrode, which enables an
unnecessary extraction current to be suppressed. The term "emitter side
surface" means a surface parallel to an emission direction of the
electron beam.

[0040]An electron beam generating apparatus related to the invention is an
electron beam generating apparatus comprising: an emitter having the
conical end sharpened in the shape of a cone on the emission side of an
electron beam; and two electrodes applying an electric field to the
conical end of the emitter, wherein the electric field is applied to the
conical end to thereby emit electrons using a Schottky effect, so that an
electron beam is generated,

[0041]the electron beam generating apparatus having an improvement that a
radius of curvature of the conical end is 1 μm or more.

[0042]the electron beam generating apparatus having an improvement that
the axial distance of the emitter tip from the suppressor electrode (the
protrusion length) is selected from the combined range of the radius of
curvature and the axial distance of the emitter tip from the suppressor
electrode, where range of values affords a desired electric field in
which said emitter is located.

[0043]According to the electron beam generating apparatus related to the
invention, since a radius of curvature of the conical end is 1 μm or
more, a focal length can be longer as compared with a conventional case
where a radius of curvature is in the range of 0.5 μm and not more
than 0.6 μm and, furthermore, the obtained angular current density can
be higher than values obtained under conventional set-ups. In addition,
since the emitter tip field is kept high enough by selecting an
appropriate protrusion length in accordance with the tip radius, the
cathode current density and the beam brightness can be maintained.

[0044]An electron beam device comprising the electron beam generating
apparatus related to the invention is a device using electron beam
generating apparatus including: an emitter having the conical geometry
end sharpened in the shape of a cone on the emission side of an electron
beam; and two electrodes applying an electric field to the conical end of
the emitter, wherein the electric field is applied to the conical end to
thereby emit electrons using a Schottky effect, so that an electron beam
is generated, the electron beam generating apparatus having an
improvement that

[0045]a radius of curvature of the conical end is 1 μm or more,

[0046]the device further comprising:

[0047]processing means conducting a predetermined processing based on an
electron beam generated by the electron beam generating apparatus.

[0048]According to the device using electron beam generating apparatus
related to the invention, since a radius of curvature of the conical end
is 1 μm or more, a focal length can be longer than a conventional case
where a radius of curvature is in the range of 0.5 μm and not more
than 0.6 μm and an angular current density can be higher than
conventional. In addition, since an angular current density is higher
than conventional, the brightness hardly decreases with beam current, so
that the processing means can conduct a predetermined processing using an
electron beam with a high brightness, thereby enabling the processing
means to be applied to various devices.

[0049]An example of devices using electron beam generating apparatus
related to the invention is an electron probe microanalyzer conducting an
analysis or observation of a specimen, and the processing means conducts
an analysis or observation of a specimen by irradiating the specimen with
an electron beam to obtain an X-ray image based on X-rays generated from
the specimen, or by irradiating a specimen with an electron beam to
obtain an electron beam image based on secondary electrons or reflected
electrons generated from the specimen.

[0050]An electron probe microanalyzer is optimal for analyzing or
observing a small region of a specimen.

[0051]Another example of the device using electron beam generating
apparatus related to the invention is an X-ray tube, and the processing
means is a target generating X-rays by collision with an electron beam.

[0052]Since the X-ray tube is equipped with an emitter emitting an
electron beam with a high brightness, an angle of an electron beam when a
target is irradiated therewith can be suppressed to be small, thereby
enabling an X-ray generating area on the target to be small. Therefore, a
spatial resolution of an X-ray image can be improved.

[0053]Still another example of the device using electron beam generating
apparatus related to the invention is an electron beam lithography
system, and the processing means conducts lithography using an electron
beam.

[0054]Since an electron beam lithography system is equipped with an
emitter emitting an electron beam with a high brightness, an angle of an
electron beam converging at one point on a pattern used in lithography
can be suppressed to be small, thereby, enabling a spatial resolution of
a pattern for lithography formed on a target of lithography to be better.

[0055]An example of the electron beam generating apparatus or the device
using electron beam generating apparatus of the invention is as follows:
a suppressor electrode and the emitter are disposed so that a protrusion
length, that is a length to the topmost point of the conical end from the
suppressor electrode, is in the range of 200 μm or more and 1500 μm
or less, and a radius of curvature of the conical end is in the range of
1 μm or more and 4 μm or less. Since a radius of curvature is in
the range of 1 μm or more and 4 μm or less and a protrusion length
is in the range of 200 μm or more and 1500 μm or less, an angular
current density can be higher than in a conventional practice and, at the
same time, an electric field at the conical end can be controlled.

[0056]In a electron beam generating apparatus or a device using electron
beam generating apparatus, the emitter preferably has a form not to
reveal a (100) crystal plane in the emitter side surface portion on the
emission side outward from a suppressor electrode that is located on the
side opposite the emission side among the two electrodes applying an
electric field, and applied with a negative voltage.

[0057]In this case, since unnecessary (100) planes are hidden in the rear
part on the side opposite the emission side of the suppressor electrode,
an unnecessary extraction current is suppressed.

[0058]An emitter related to the invention is an emitter generating an
electron beam in which the conical end of an emitter on the emission side
of an electron beam is sharpened in the shape of a cone and is applied
with an electric field to thereby emit electrons using a Schottky effect,
the emitter having an improvement that

[0059]a radius of curvature of the conical end is 1 μm or more.

[0060]According to an emitter related to the invention, since a radius of
curvature of the conical end is 1 μm or more, a focal length can be
longer than a conventional practice where a radius of curvature is in the
range of from 0.5 μm to not more than 0.6 μm and an angular current
density can be higher than conventional.

[0061]In a case where a protrusion length of an emitter is adjusted in the
range of 200 μm or more and 1500 μm or less with a radius of
curvature in the range of 1 μm or more and 4 μm or less according
to the proposed combined range of radius curvature and the protrusion
length which ensures high enough tip electric field (see FIG. 4), the
beam brightness can be kept high.

[0062]An emitter related to the invention preferably has a form not to
reveal a (100) crystal plane in the emitter side surface portion.

[0063]In this case, unnecessary (100) crystal planes are hidden in the
rear part on the side opposite the emission side of the suppressor
electrode, which enables an unnecessary extraction current to be
suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064]For the purpose of illustrating the invention, there are shown in
the drawings several forms of which are presently preferred, it being
understood, however, that the invention is not limited to the precise
arrangement and instrumentation shown.

[0065]FIG. 1A is a schematic diagram showing a Schottky emitter related to
one example of the invention;

[0066]FIG. 1B is an enlarged schematic diagram of the conical end of the
emitter;

[0067]FIG. 1C is an enlarged diagram for comparison with the conical end
of a conventional emitter;

[0068]FIG. 2 is a schematic diagram of an electron beam generating
apparatus equipped with the Schottky emitter;

[0069]FIG. 3 is a schematic block diagram of an electron probe
microanalyzer (EPMA) equipped with the electron beam generating
apparatus;

[0070]FIG. 4 is a graph roughly showing a combined range of a protrusion
length and a radius of curvature;

[0071]FIG. 5A is a schematic diagram showing a configuration of a
suppressor electrode and a Schottky emitter processed by means of a DC
etching method or a similar means to produce similar shapes;

[0072]FIG. 5B is a schematic diagram showing a configuration of a
suppressor electrode and a Schottky emitter processed by means of an AC
etching method or other means to produce similar shapes;

[0073]FIG. 6 is a graph showing relationships between a beam current value
and a reduced brightness in an example of the invention, a standard
Schottky emitter of a conventional technology, and a tungsten filament
emitter;

[0074]FIG. 7 is a schematic block diagram of a microfocus X-ray tube
equipped with a Schottky emitter;

[0075]FIG. 8 is a schematic block diagram of an electron beam exposure
system equipped with a Schottky emitter;

[0076]FIG. 9A is a diagram schematically showing an electron source
characteristics when an electron beam is emitted from an electron gun of
a thermionic emitter;

[0077]FIG. 9B is a diagram schematically showing an electron source
characteristics when an electron beam is emitted from an electron gun of
a field emitter;

[0078]FIG. 10 is a graph showing relationships between a beam current
value and brightness in a conventional Schottky emitter and a tungsten
filament emitter;

[0080]FIG. 11B is an enlarged schematic diagram of the conical end of the
emitter;

[0081]FIG. 12 is a diagram schematically showing an emitter of an electron
gun and the definition of the electron gun focal length; and

[0082]FIG. 13 is a graph describing the findings and knowledge leading to
the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0083]Detailed description will be given of a preferred embodiment of the
invention below based on the accompanying drawings.

[0084]FIG. 1A is a schematic diagram showing a Schottky emitter related to
one example of the invention. FIG. 1B is an enlarged schematic diagram of
the conical end of the emitter. FIG. 1C is an enlarged diagram for
comparison with the conical end of a conventional emitter. FIG. 2 is a
schematic diagram of an electron beam generating apparatus equipped with
the Schottky emitter. FIG. 3 is a schematic block diagram of an electron
probe microanalyzer (EPMA) equipped with the electron beam generating
apparatus.

[0085]A Schottky emitter 1 related to the example, as shown in FIG. 1A,
has the conical end 1a sharpened in the shape of a cone on the emission
side of an electron beam B (here, the symbol "B" in the figure denotes
the electron beam and should not be confused with the quantity
representing the brightness). The Schottky emitter 1 has a construction
in which a zirconium oxide layer is coated on a single crystal wire of
tungsten. As shown in FIG. 1B, a radius of curvature R of the conical end
1a is 2.0 μm and preferably in the range of 1 μm or more, which is
larger as compared with a conventional conical end 201a in the range of
0.5 μm and not more than 0.6 μm (see FIG. 11B and FIG. 1C). Note
that in FIG. 1C, the conical end 1a of a Schottky emitter 1 related to
the example is shown with a two-dot chain line.

[0086]An electron beam generating apparatus 10 equipped with the Schottky
emitter 1, as shown in FIG. 2, includes two electrodes 2 and 3 applying
an electric field to the conical end 1a of the Schottky emitter 1; an
anode 4 extracting an electron beam B; and a condenser lens 5 converging
the electron beam B. A portion consisting of the Schottky emitter 1 and
the electrodes 2 and 3 (a suppressor electrode 2 and an extractor
electrode 3 described later) is called an electron source. The electron
source is easy to be understood with a potential of the Schottky emitter
1 as a reference (in FIG. 2, the potential is 0 V. In an actual case, a
potential of the emitter 1 is usually at a negatively high potential).
The conical end 1a is heated under a strong electric field applied
thereto by the electrodes 2 and 3 to thereby emit electrons using a
Schottky effect and the electron beam B is thus generated by the electron
beam generating apparatus 10. The electron beam generating apparatus 10
corresponds to an electron beam generating apparatus of the invention and
also corresponds to an electron beam generating apparatus.

[0087]Of the two electrodes 2 and 3, the electrode 2 located on the side
opposite the emission side and applied with a negative voltage (in FIG.
2, -300 V) is a suppressor electrode 2 and the electrode 3 located on the
emission side and applied with a positive voltage (in FIG. 2, 6423 V) is
an extractor electrode 3.

[0088]The anode 4 is disposed opposite the Schottky emitter 1 serving as a
cathode and applied with a positive voltage with respect to the emitter
1. The anode 4 attracts the electron beam B emitted from the Schottky
emitter 1. The electron beam B is accelerated by attraction by the anode
4.

[0089]The condenser lens 5 is constructed in the shape of a ring. A
current is supplied into the condenser lens 5 from a lens power supply
not shown to thereby generate a magnetic field to converge the electron
beam B in a similar way to light in an optical condenser lens.

[0090]Description will be given of a configuration of the Schottky emitter
1, the electrodes 2 and 3, the anode 4 and the condenser lens 5 in the
electron beam generating apparatus 10, again, with reference to FIG. 2.
The suppressor electrode 2 and the extractor electrode 3 are disposed
with a spacing of 700 μm therebetween. Even though the spacing is
similar to the conventional Schottky emitter configurations, the
disposition of each electrode is unique. The length from the suppressor
electrode 2 to the topmost point of the conical end 1a is indicated by
Lst. On the other hand, a length to the extractor electrode 3 from
the topmost point of the conical end is indicated by LTE. Hence, a
relation that Lst+LTE=700 μm is established. If the emitter
is operated with the same protrusion length LST of 250 μm as
conventional, an electric field strength F at the conical end 1a cannot
be secured to be a necessary value (in this case, F=1×109
V/m). Therefore, a protrusion length LST is set to be longer than
conventional case in order to raise an electric field strength F to a
necessary value (1×109 V/m). In a case of a Schottky emitter
where a radius of curvature R of the conical end 1a is 2.0 μm, the
Schottky emitter 1 and the suppressor electrode 2 are disposed so that a
protrusion length LST is 400 μm. Therefore, LTE is 300 μm
(=700 μm-LST).

[0091]In order to secure a necessary electric field strength F at the
conical end 1a, a protrusion length LST is adjusted so as to be
adapted for a radius of curvature R. That is, a characteristic of an
electric field versus a protrusion length LST also varies depending
on a radius of curvature R. Hence, as shown in FIG. 4, a combined range
of a protrusion length LST and a radius of curvature R are set in
advance based on a value of an electric field necessary for field
emission in the Schottky mode. In this case, a protrusion length LST
and a radius of curvature R are individually altered to estimate the
combined range suitable for the necessary electric field strength F
(1×109 V/m) (see a crosshatched portion in FIG. 4). That is, a
combined range of a radius of curvature R and a protrusion length
LST necessary for an operation of the Schottky emitter 1 is defined
by the hatched area in FIG. 4. Note that in FIG. 4, there is shown a
combination of a radius of curvature R and a protrusion length LST
(where R=0.5 μm and LST=250 μm) of a conventional standard Schottky
emitter (with R=0.5 μm) with a mark "x".

[0092]A distance between the anode 4 and the condenser lens 5 is indicated
by L. In a case of a thermionic emitter, the anode 4 and the condenser
lens 5 are spaced with a distance of a value of the order of L=100 mm.
Though L is longer and a lens aberration coefficient is larger, the
problem of larger aberrations leading to larger beam diameter is not
incurred since a thermionic emitter has a large angular current density.
Contrast thereto, in a case of a Schottky emitter, since an angular
current density is smaller, the intrinsically high brightness of the
Schottky emitter is degraded by an increase in lens aberration
coefficients. Hence, in a case of a Schottky emitter, it is preferable
that in order to suppress a lens aberration coefficient, L is set to be
as close to 0 mm as possible to thereby locate the condenser lens 5 so as
to be closer to the side of the Schottky emitter 1.

[0093]If a Schottky emitter 1 is processed using a direct current (DC)
etching method or other suitable means, unnecessary (100) crystal planes
are revealed, as shown in FIG. 5A, forward from the suppressor electrode
2, that is on the emission side (see hatching with oblique lines inclined
to the right in the figure). A work function of a (100) crystal plane
decreases by the action of a zirconium oxide layer and an unnecessary
extraction current is extracted with a result of increasing a load on a
power supply. As a result, larger outgas rate, which is a gas load from
the surrounding electrode surfaces, is generated to degrade the degree of
vacuum in the vicinity of the emitter. The term, a DC etching method, is
an etching conducted without altering polarities of electrodes used in
the etching.

[0094]Contrast thereto, in a case where an alternate current (AC), or
similar means are used in forming a Schottky emitter 1, the etching can
be conducted, as shown in FIG. 5B, so that the conical end 1a in the
shape of a cone is longer with oblique lines in profile. The AC etching
method enables not only an etched macro face with oblique lines in
profile to be obtained, but also a crystal face different from a (100) to
be microscopically produced. Hence, by processing the Schottky emitter 1
with an AC etching method, a (100) is not revealed on a crystal surface
in the emitter side surface portion on the emission side outward from the
suppressor electrode 2. The term "emitter side surface" is a surface
parallel to the emission direction of the electron beam B. Therefore,
hatching with oblique lines inclined to the right in the figure indicates
(100) crystal planes in the emitter side surface portion. With such a
construction adopted, unnecessary (100) crystal planes are hidden in the
rear part on the side opposite the emission side of the suppressor
electrode 2, which enables an unnecessary extraction current to be
suppressed. The term, an AC etching method, is to conduct etching while
polarities of electrodes used for the etching are alternated.

[0095]Then, description will be given of a method for controlling an
electron beam. To begin with, a radius of curvature R of the conical end
1a is adjusted. A radius of curvature R is adjusted to a value larger
than conventional in order to control the electron gun focal length f to
be longer and to control the angular current density to be higher. Since
a radius of curvature of a conventional conical end 1a is in the range of
0.5 μm to no more than 0.6 μm, it is preferable to select a radius
of curvature R in the range of 1 μm or more. In one embodiment given
here by way of example, a radius of curvature R is selected to be 2
μm.

[0096]As described above, a combined range of a protrusion length LST
and a radius of curvature R is set in advance based on an electric field
value. The suppressor electrode 2 and the Schottky emitter 1 are disposed
by determining a protrusion length LST, which is a length from the
suppressor electrode 2 to the topmost point of the conical end of the
Schottky emitter 1. In adjustment of a protrusion length LST, a
protrusion length LST is selected from the combined range shown in
FIG. 4 at an adjusted radius of curvature R.

[0097]In FIG. 4, a desirable combined range of a radius of curvature R and
the protrusion length LST is defined in the range selected in the
range 1 μm<R<4 μm and 200 μm<LST<1500 μm. In
one embodiment shown here by way of example, a combination of a radius of
curvature R and a protrusion length LST (R=2.0 μm and
LST=400 μm) is selected. By selecting a combination point in the
combined range in FIG. 4, an electric field strength F at the conical end
1a can be controlled (in the example, F=1×109 V/m).

[0098]A focal length f or the electron beam B is controlled by a radius of
curvature R thus adjusted. An angular current density of the electron
beam B is controlled by a controlled focal length f, while the beam
brightness is maintained at its intrinsically high value by guaranteeing
a large enough tip electric field through the emitter protrusion length
adjustment.

[0099]In a case where a radius of curvature of the conical end 1a is set
in the range of 1 μm or more and 4 μm or less and a protrusion
length LST of the Schottky emitter 1 is adjusted in the range of 200
μm or more and 1500 μm or less, an angular current density can be
higher than conventional and at the same time, an electric field F at the
conical end 1a can be controlled to maintain the high beam brightness.

[0100]Since an angular current density is higher than conventional
geometry, a brightness is scarcely reduced even at relatively high beam
currents and in the EPMA 50 of the example, the elementary analysis
processing section 20 and the surface observation processing section 30
can conduct predetermined processing such as elementary analysis
processing and surface observation processing, respectively with a high
brightness electron beam. Therefore, the invention can be applied to
various apparatuses represented by such an EPMA 50.

[0101]Note that in a case where an electron beam generating apparatus 10
related to the example is used in EPMA 50, the following effect is
exerted. That is, EPMA 50 requires a beam current at a level of sub μA
or μA, and it is also confirmed in FIG. 6 that in EPMA 50, no
reduction in the brightness in the Schottky emitter 1 is observed even
with a level of sub μA or μA.

[0102]FIG. 6 is a graph showing relationships between a beam current value
and a brightness in a Schottky emitter 1 (R=2.0 μm) related to the
example, a standard Schottky emitter (R=0.6 μm) of a conventional
technology and a tungsten filament emitter as a thermionic emitter. That
is, the graph of FIG. 6 is obtained by adding the graph showing a
relationship between a beam current value and a brightness in the
Schottky emitter 1 related to the example to FIG. 10. FIG. 6 was obtained
in the same condition as in FIG. 10. In the Schottky emitter 1 related to
the example, however, the relationships were obtained in the conditions
that a current density js is 1.0×104 A/cm2, a
temperature T is 1800 K and a angular current density J.sub.ΩGSE is
2.22 mA/str. A curve drawn with a dotted line is a curve of the tungsten
filament emitter and two curves drawn with a solid line in FIG. 6 is a
curve of the Schottky emitter, wherein "Giant SE" in the graph indicates
the curve of a Schottky emitter 1 (R=2.0 μm) related to the example
having a radius of curvature R of the conical end 1a larger than
conventional and "Standard SE" indicates the curve of a conventional
standard Schottky emitter (R=0.6 μm). The mark of "W filament" in the
graph indicates a tungsten filament.

[0103]It is found from FIG. 6 that in the case of a conventional standard
Schottky emitter, an angular current density is low and an electron
source diameter is small; therefore, a brightness begins to decrease at a
beam current in the vicinity of 1 nA or greater and reduces by as much as
6 orders of magnitude at a level of 1 μA. Contrast thereto, in a case
of a Schottky emitter 1 related to the example, an angular current
density is high; therefore, it has been confirmed that a brightness is
hard to decrease as compared to a standard Schottky emitter and a
brightness decrease starts at about 1 μA if a position of the
condenser lens 5 is properly selected. Hence, a Schottky emitter can be
applied to a device requiring a beam current at a level of sub μA or
μA such as in an EPMA 50.

[0104]The invention can be modified in the following way without limiting
to the embodiment.

[0105](1) In the example, description was given of an electron probe
microanalyzer (EPMA) as an example of a device using an electron beam
generating apparatus, no specific limitation is imposed on a device as
far as an electron beam generating apparatus is used therein. For
example, the device may be a scanning electron microscope (SEM), a
transmission electron microscope (hereinafter also referred to as "TEM"
for short), a microfocus X-ray tube, an Auger electron spectrometer, an
electron beam lithography system and an electron beam writer. A
transmission electron microscope (TEM) can observe a projected image by
causing an electron beam to be transmitted through a thin film specimen
with a thickness of the order in the range of several tens to hundreds of
nanometres. A microfocus X-ray tube generates an X-ray beam with a small
diameter of the order in the range of from sub μm to several μm by
causing an electron beam to collide with a target. An Auger electron
spectrometer examines energy of Auger electrons to conduct an elementary
analysis on a specimen. An electron beam lithography system conducts
lithography with an electron beam instead of light in a conventional
technology. An electron beam writer produces "masters" for high density
optical disks.

[0106]Description will be given not only of the microfocus X-ray tube but
also of the electron beam exposure system as an example of electron beam
lithography system. FIG. 7 is a schematic block diagram of a microfocus
X-ray tube and FIG. 8 is a schematic block diagram of an electron beam
exposure system.

[0107]A microfocus X-ray tube 70 equipped with an electron beam generating
apparatus 10, as shown in FIG. 7, includes a target 60 generating X-rays
by collision of an electron beam therewith. The electron beam generating
apparatus 10 is equipped with not only the suppressor electrode 2, the
extractor electrode 3, the anode 4 and the condenser lens 5, but also an
iris lens 6 and an objective lens 7. The iris lens 6 has an aperture 6a
having a diameter reducing hole that defines converging angle of an
electron beam B. The condenser lens 5, the iris lens 6, the objective
lens 7 and the target 60 are sequentially disposed in order from the
upstream side (the emitter 1 side) to the downstream side in an
irradiation direction of the electron beam B. The target 60 is formed
from a material generating X-rays represented by tungsten. The target 60
corresponds to processing means of the invention.

[0108]Since the microfocus X-ray tube 70 has a Schottky emitter 1 in which
an electron beam brightness does not deteriorate at a high beam current
condition, an angle of the electron beam B when the target 70 is
irradiated with the electron beam B can be suppressed to be small,
thereby enabling the electron beam size focused on the target to be
small. Consequently, an X-ray generating region on the target 60 can be
smaller, and a spatial resolution of an X-ray image is improved.

[0109]The electron beam exposure system 90 equipped with the electron beam
generating apparatus 10 includes an exposure processing section 80
conducting exposure on a substrate W as shown in FIG. 8. The electron
beam generating apparatus 10 is equipped with: the suppressor electrode
2; the extractor electrode 3; the anode 4; and the condenser lens 5. The
exposure processing section 80 is equipped with: irradiation lens 81,
image forming lens 82; a shaping aperture 83; a blanker 84; a reticle 85;
and a contrast aperture 86. The reticle 85 is an original drawing of an
exposure pattern. The exposure processing section 80 corresponds to
processing means of the invention.

[0110]In the electron beam exposure system 80 shown in FIG. 8, each pair
of lenses 5, 81 and 82 is disposed one on the other. Not only is the
shaping aperture 83 disposed between the condenser lens 5 on the
downstream side and the irradiation lens 81 in the upstream side in the
irradiation direction of the electron beam B, but the blanker 84 is
disposed between the irradiation lens 81 on the upstream side and the
irradiation lens 81 on the downstream side in the irradiation direction
of the electron beam B. Not only is the reticle 85 disposed between the
irradiation lens 81 on the downstream side 81 and the image forming lens
82 on the upstream side, but the contrast aperture 86 is disposed between
the image forming lens 82 on the upstream side and the image forming lens
82 on the downstream side.

[0111]Since the electron beam exposure system 90 is equipped with a
Schottky emitter 1 emitting an electron beam B high in brightness, an
angle of the electron beam B converging to one point on the reticle 85
can be suppressed to be small, thereby enabling a spatial resolution of
an exposure pattern focused on the substrate W to be improved.

[0112](2) In the example, an AC etching method is adopted to form the
Schottky emitter 1 so as not to reveal a (100) crystal plane in the
emitter side surface portion on the emission side outward from the
suppressor electrode 2, while no limitation is placed on an AC etching
method as far as a (100) crystal plane is not revealed in the emitter
side surface portion.

[0113](3) In the example, a Schottky emitter 1 has a shape where no (100)
crystal plane is revealed in the emitter side surface portion on the
emission side outward from the suppressor electrode 2, while the Schottky
emitter 1 is not necessarily required to have a shape shown in FIG. 5B
unless an unnecessary extraction current is suppressed. For example, as
shown in FIG. 5A, unnecessary (100) crystal planes may be revealed
forward from the suppressor electrode 2.

[0114]The invention may be embodied in other specific forms without
departing from the spirit or essential attributes thereof and,
accordingly, reference should be made to the appended claims, rather than
to the foregoing specification, as indicating the scope of the invention.

[0115]In this specification, the verb "comprise" has its normal dictionary
meaning, to denote non-exclusive inclusion. That is, use of the word
"comprise" (or any of its derivatives) to include one feature or more,
does not exclude the possibility of also including further features.

[0116]All of the features disclosed in this specification (including any
accompanying claims, abstract and drawings), and/or all of the steps of
any method or process so disclosed, may be combined in any combination,
except combinations where at least some of such features and/or steps are
mutually exclusive.

[0117]Each feature disclosed in this specification (including any
accompanying claims, abstract and drawings), may be replaced by
alternative features serving the same, equivalent or similar purpose,
unless expressly stated otherwise. Thus, unless expressly stated
otherwise, each feature disclosed is one example only of a generic series
of equivalent or similar features.

[0118]The invention is not restricted to the details of the foregoing
embodiment(s). The invention extends to any novel one, or any novel
combination, of the features disclosed in this specification (including
any accompanying claims, abstract and drawings), or to any novel one, or
any novel combination, of the steps of any method or process so
disclosed.